Spin coating method and apparatus for liquid carbon dioxide systems

ABSTRACT

A spin coating method comprises applying a carbon dioxide liquid to surface portion of a substrate; and then rotating the substrate about an axis to distribute the carbon dioxide liquid on the substrate. The carbon dioxide liquid can be distributed on the substrate as a carrier, for the purpose of depositing a material such as a coating on the substrate. In addition, the carbon dioxide liquid can be distributed on the substrate as a solvent, for the purpose of solubilizing, dissolving or removing a material previously deposited on the surface of the substrate. Apparatus for carrying out the present invention is also disclosed.

FIELD OF THE INVENTION

The present invention relates to spin coating methods and apparatus inwhich the need to use volatile organic solvents to carry or dissolve thecoating material is obviated by the use of liquid carbon dioxide.

BACKGROUND OF THE INVENTION

The microelectronics industry is dependent upon spin coating for thedeposition of uniform thin films on various flat substrates, includingsilicon wafers for integrated circuits and glass for optoelectronicdevices. Spin coating processes typically involve the injection of aliquid coating onto a flat substrate that is rotating at a high rate ofspeed such as several hundred to several thousand RPM. A spin coateroperates by spreading a thin liquid layer (liquid coating solution) on asubstrate using the centrifugal force of a spinning substrate. Thespreading liquid layer then forms a fairly uniform film afterevaporation that covers up and planarizes topological unevenness leavingpin-hole free films. The liquid coating solution is typically comprisedof a material that has low volatility such as a polymeric material or anoligomer. These low volatile materials are then dissolved in a volatileliquid solvent or liquid solvent mixture. The volatile liquid solventsare typically volatile organic solvents (VOC), chlorofluorocarbons(CFC), hydrochlorofluorocarbons (HCFC), hydrofluorocarbons (HFC), andperfluorocarbons or mixtures thereof such as methyl ethyl ketone (MEK),ethyl acetate, chloroform, toluene, isoamyl acetate, Freon-113,Freon-22, Freon-134a, Freon-227, perfluoromorpholine, etc. The liquidcoating solution then thins in a fairly uniform matter across thespinning flat substrate and the excess liquid coating solution falls offthe edge of the substrate and is collected. The thin film of liquidcoating solution that is retained on the rotating flat substratecontinues to evaporate leaving behind a uniform thin film of thecompound that has low volatility. The film typically is comprised of apolymeric material (such as a photoresist, interlayer dielectric) butother materials can also be deposited in this way such as low molecularweight compounds (adhesion promoters, antireflective coatings) orsol-gel precursors.

Sometimes a spin coating apparatus is used to remove a substance from asubstrate. For example, to develop an exposed photoresist pattern, aliquid solvent is injected onto a substrate that is or will be rotatingat a high rate of speed. This liquid solvent can then dissolve thedesired substance off the rotating substrate effectively removing thesubstance off of the rotating substrate.

Spin coating is a solvent intensive process and accounts for a largeportion of the use of solvents by the microelectronics industry.Accordingly, there remains a need for spin coating methods and apparatusthat reduce or eliminate the use of VOC, CFC, HCFC, HFC, or PFCsolvents.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a spin coating apparatus forapplying a liquid to a substrate. The apparatus comprises a pressurevessel having an enclosed chamber therein; a turntable for rotating asubstrate positioned in the enclosed chamber; a liquid dispenserpositioned in the enclosed chamber for depositing a carbon dioxideliquid on that substrate; and a pressure reservoir in fluidcommunication with the liquid dispenser for storing the carbon dioxideliquid. The apparatus typically includes a pressure controlleroperatively associated with the chamber that can maintain a pressurewithin the chamber at about 100 to 10,000 psi.

A second aspect of the present invention is a spin coating method. Themethod comprises applying a carbon dioxide liquid to surface portion ofa substrate; and then rotating the substrate about an axis to distributethe carbon dioxide liquid on the substrate. The carbon dioxide liquidcan be distributed on the substrate as a carrier, for the purpose ofdepositing a material such as a coating on the substrate. In addition,the carbon dioxide liquide can be distributed on the substrate as asolvent, for the purpose of solubilizing, dissolving or removing amaterial previously deposited on the surface of the substrate.

The foregoing and other objects and aspects of the present invention areexplained in greater detail in the drawings herein and the specificationset forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an apparatus of the present invention.

FIG. 2 is a side view of a second embodiment of an apparatus of thepresent invention that maintains a differential partial pressure ofcarbon dioxide between carbon dioxide in the chamber atmosphere andcarbon dioxide liquid deposited on the substrate.

FIG. 3 is a schematic view of a third embodiment of an apparatus of thepresent invention, including a robotic device for exchanging substrateswithout opening the main chamber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Carbon dioxide is a gas at standard pressures and temperatures. Onefeature of a spin coating apparatus of the present invention is,accordingly, that the carbon dioxide system is provided to the substrateas a liquid. This is necessary because the liquid must spread on thespinning substrate and the volatile components must evaporate from thesubstrate leaving behind the non-volatile film-forming material. Wherethe carbon dioxide is utilized as a solvent, this is also necessary toprevent the carbon dioxide from evaporating too quickly to remove thecompound to be removed from the substrate.

An apparatus for carrying out the present invention is illustrated inFIG. 1. The apparatus comprises pressure vessel 10 having an enclosedchamber 11 therein. The workpiece is a substrate 12 that is positionedin the enclosed chamber for treatment. Any suitable substrate can beemployed, including but not limited to glass, ceramic, semiconductormaterials such as silicon wafers, compact discs, etc. In general, thesubstrates have a planar top surface portion to be treated. A turntable13 that rotates the substrate is positioned within the enclosed chamber,with the turntable being comprised of a holder 14 and a spinningmechanism (e.g., a motor or motor and drive system) 15. Any spinningmechanism can be employed, with speeds of about 10 to 10,000 revolutionsper minute being typical. The holder illustrated holds the substrate bydifferential pressure leads, and a differential pressure control 16,thus serving as a holding or retaining means for securing the substrateto the turntable while the substrate is spun about the turntable axis,but the substrate could be held in place by any suitable means, such aspins, clamps, adhesive, etc. Multiple substrates can be provided on asingle turntable, as described in U.S. Pat. No. 4,640,846 (thedisclosures of all U.S. Patent references cited herein are to beincorporated herein by reference).

A liquid dispenser 17 is positioned in the enclosed chamber above thesubstrate so that it can deposit a carbon dioxide liquid on thesubstrate through valve 18 to nozzle 19. A pressure reservoir 30 forstoring a carbon dioxide liquid is located outside of the chamber and isin fluid communication with the liquid dispenser by line 31, undercontrol of valve 32. A pressure pump 40 (e.g., a syringe pump or adiaphragm pump) is connected to the chamber by line 41 and valve 42 toelevate the pressure within the chamber to a suitable operatingpressure, as discussed below.

A pressure transducer 50 is connected to the chamber by line 51. Thepressure transducer is operatively associated with a pressure controller52 for controlling valve 53, which allows the atmosphere within thechamber to be vented in a controlled manner as needed by line 54. Aswill be appreciated, since the chamber is a closed system, once aninitial pressure is achieved by charging the chamber with pressure pump40, as additional material, particularly carbon dioxide liquid, isprovided into the chamber, pressure may need to be released by ventingthrough line 54 to maintain the desired pressure.

A pool of carbon dioxide liquid 60 is provided in the chamber, whichpool may be drained by line 61 and valve 62. Such a pool may be used tocontrol the differential pressure of the carbon dioxide between theliquid deposited on the substrate and the atmosphere in the chamber, asdiscussed below.

If desired, the liquid dispenser 17 can dispense carbon dioxide liquid,and the polymer or other compound to be mixed with the liquid anddeposited on the substrate can be separately dispensed onto thesubstrate where it is thereon mixed with the liquid. In addition, theliquid dispenser nozzle may be modified to control the shape or rate atwhich the liquid is dispensed, as described in U.S. Pat. No. 5,392,989to Hurtig.

In operation, a carbon dioxide liquid is applied to the top surfaceportion of the substrate 12 from the dispenser 17, and the substrate isrotated about an axis (which axis may or may not pass through thesubstrate) to distribute the carbon dioxide liquid on the top surfaceportion of the substrate. If desired, the liquid may be dispensed on thesubstrate and allowed to sit, or remain stationary thereon, for a periodof time before spinning is initiated, as described in U.S. Pat. No.4,281,057 to Castellani et al.

Either or (preferably) both of the applying step and the rotating stepare carried out with the pressure in the chamber greater thanatmospheric pressure: in general, with the pressure in the chamber beingelevated by at least 10 or 20 pounds per square inch (psi) up to 5,000or 10,000 psi. The temperature in the chamber is typically maintained atfrom -53 or -20° C. up to about 20 or 30° C. In addition, as discussedin greater detail below, either (or both) of the applying and spinningsteps are carried out (in whole or in part) in an atmosphere comprisingcarbon dioxide (and other ingredients such as inert gases) whilemaintaining a differential partial pressure of carbon dioxide betweenthe carbon dioxide liquid applied to the surface portion of thesubstrate and that atmosphere (particularly the atmosphere immediatelyabove the top surface portion of the substrate) of between about 10, 20or 40 mm Hg to about 100, 200 or 400 mm Hg.

It may be desired to maintain an essentially constant atmosphere withinthe chamber as carbon dioxide liquid is applied during one (or more)application and spinning steps. In this case, an atmosphere of carbondioxide and one or more additional gasses or inert gasses (e.g., helium,nitrogen, argon, oxygen) can be passed into the chamber via line 41above and vented from the chamber via line 54 above in a controlledmanner, so that the composition of the atmosphere within the chamberremains consistent as material is added to the chamber via dispenser 17.

The carbon dioxide liquid, when used to remove previously appliedmaterials from the substrate, may consist essentially of carbon dioxide,with minor ingredients such as one or more cosolvents (e.g.,tetrahydrofuran, cyclohexane, alcohols such as methanol or ethanol,etc).

When used as a carrier for a compound to be distributed and deposited onthe top surface portion of the substrate, the carbon dioxide liquid is amixture that contains carbon dioxide, optionally one or more cosolvents,and one or more compounds to be carried. Exemplary compounds that may becarried by the carbon dioxide liquid include, but are not limited to,polymers (including polymer precursors or monomers that polymerize orare polymerized after deposition), resists (e.g., photoresists, electronresists, x-ray resists), adhesion promoters, antireflective coatings,and sol-gel precursors. Resists such as photoresists may also containadditives to improve lithographic performance including dissolutioninhibitors, photo acid generators, and the like. The photo acidgenerators are present to allow for chemically amplified resisttechnology. The mixture may be in any physical form, includingsolutions, dispersions, and emulsions, but preferably the mixture is asolution. In a preferred embodiment, the mixture is comprised of carbondioxide and a fluoropolymer, and more preferably a fluoroacrylatepolymer. Examples of such mixtures are disclosed as the polymerizationproduct described in U.S. Pat. No. 5,496,901 to DeSimone, the disclosureof which is incorporated herein by reference. In a preferred embodiment,such mixtures are applied to the top surface portion of a semiconductor(e.g., silicon) substrate substrate to serve as a photoresist.

The carbon dioxide liquid may contain a viscosity modifier such as anassociative polymer to increase the viscosity thereof and alter thethicknesss of the surface coating. The viscosity modifier may, forexample, be included in an amount sufficient to increase the viscosityof the carbon dioxide liquid up to about 500 or 1000 centipoise.

The carbon dioxide liquid may contain a surface tension modifier (e.g.,a surfactant) to increase or decrease the surface tension by an amountup to about plus or minus 5 dynes per centimeter. Surface tensionmodifiers may be included to increase or decrease droplet formation atthe boundary of the carbon dioxide liquid coating formed on thesubstrate during spinning thereof. Surfactants used as such surfacetension modifiers should include a CO₂ -philic group and a CO₂ -phobicgroup and are known in the art. See, e.g., U.S. Pat. No. 5,312,882 toDeSimone et al.; U.S. Pat. No. 5,683,977 to Jureller et al. (thedisclosures of which are incorporated by reference herein in theirentirety).

If desired, the carbon dioxide liquid may contain a co-solvent thatevaporates more slowly than does carbon dioxide (e.g., alcohols, ketonessuch as cyclopentanone, butyl acetate, xylene). Substrates coated withsuch a carbon dioxide liquid may then be removed from the pressurevessel and dryed (e.g., in a separate drying oven, as described in U.S.Pat. No. 4,794,021 to Potter). Such a technique may be employed toreduce pin holes in the coating formed on the substrate.

The apparatus disclosed in FIG. 2 is similar to the apparatus disclosedin FIG. 1, and includes a pressure vessel having an enclosed chambertherein, a substrate, a turntable comprised of a holder and a spinningmechanism, differential pressure leads, and a liquid dispenserpositioned in the enclosed chamber above the substrate so that it candeposit a carbon dioxide liquid on the substrate through valve 18 tonozzle 19. A pressure reservoir 30 for storing a carbon dioxide liquidis located outside of the chamber and is in fluid communication with theliquid dispenser by line, under control of valve. A pump is againconnected to the chamber by line and valve to elevate the pressurewithin the chamber. A liquid carbon dioxide pool 60 in the pressurevessel is supplied with a carbon dioxide liquid by lines 70 and hascooling coils 72 placed therein. A temperature controller (not shown) isprovided for the cooling coils to maintain a temperature differencebetween a carbon dioxide liquid in the pool and the carbon dioxideliquid deposited on the substrate (which may be determined by aninfra-red monitor or other suitable temperature sensor positioned withinthe chamber and directed at the top surface of the substrate, andoperatively associated with the temperature controller). This serves asa differential pressure control means for maintaining a differentialpartial pressure of carbon dioxide between the carbon dioxide liquidapplied to the surface portion of the substrate and that atmosphere, asdiscussed above. In a preferred embodiment, where only a 0.01 or 0.05 to0.1 or 0.5 atmosphere partial pressure difference is required, thetemperature is controlled to plus or minus 1 or 0.1 degrees Centigrade,with the temperature of the liquid in the pool being cooler than that ofthe liquid on the substrate. It should also be noted that, as analternate to sensing temperature, a different parameter, such as filmthickness of the carbon dioxide liquid on the surface of the substrate,may be measured, and the partial pressure of carbon dioxide regulated inresponse thereto.

An alternate means for maintaining a differential partial pressure ofcarbon dioxide is also provided in FIG. 2 in the form of cooling coils75 for cooling the walls of the chamber to cause some condensation tooccur on the walls of the chamber, and thus remove carbon dioxide fromthe vapor phase above the spinning substrate.

A still additional means for maintaining a differential partial pressureis to equilibrate the atmosphere with liquid carbon dioxide at the sametemperature as the liquid being deposited, but add very small flow ratesof an inert gas via innert gas supply line 41 at the same time that gasand vapor are removed from the chamber by fan in order to lower thepartial pressure of carbon dioxide. In the illustrated embodiment,pressure vessel 75 contains a carbon dioxide liquid pool, and issupplied with an inert gas (e.g., nitrogen) via line 76 which passesthrough the carbon dioxide pool so that the inert gas is provided in acarbon dioxide-saturated atmosphere.

As will be apparent to those skilled in the art, the methods andapparatus of the present invention may employ any one or any combinationof such means for maintaining a partial pressure of carbon dioxide.

FIG. 3 is a schematic view of an automated multiple substrate spincoating station. The device includes a pressure vessel and a turntable,essentially as described above. A robotic arm 90 is equipped with arobotic grasping member 91. A substrate shuttle car 92, shown in themain chamber, is provided on a pair of alignment rails 93, 94. Shuttlecars are moved by drive systems in accordance with standard techniques.An an ante chamber 95 (which serves as a pressure lock) outside the mainchamber provides a starting point for the shuttle car. A receivingchamber 96 (containing a second shuttle car as illustrated; also servesas a pressure lock) is provided for receiving a shuttle car after allsubstrates on that shuttle car have been treated. Both the ante-chamberand receiving chamber are provided with separate internal and externalsealable doors (not shown), which doors enable both chambers to serve aspressure locks while the shuttle cars are inserted and removed so thatthe main chamber need not be fully depressurized to insert a new shuttlecar. A removable fluid splash barricade 97 around the turntable preventsexcess liquid from splashing onto the shuttle car, substrates, androbotic arm. The fluid barricade, robotic arm, grasping member, andshuttle cars may all be driven by standard drive techniques and theoperation thereof coordinated with a controller, such as a programmablecomputer, in accordance with standard techniques. Together, the roboticarm and shuttle car serve as an exchanging exchanging means positionedwithin the enclosed chamber for exchanging a first substrate with asecond substrate on said turntable without the need for opening saidpressure vessel. As will be appreciated, the exchanging means need nothave a shuttle car and pressure lock system as shown, and may insteadhave one or more stationary racks inside the main chamber, or may employa single pressure lock.

The foregoing is illustrative of the present invention, and is not to beconstrued limiting thereof For example, means other than those describedabove may be employed for handling the substrates while minimizingpressure loss from the pressure chamber. By way of example, theextendible robotic arm may be mounted on a track or framework of tracksconfigured to move the grasping member between the substrate holder andthe shuttle car. The shuttle car may itself be eliminated, the arm andassociated movement mechanism(s) being adapted to move the graspingmember between the ante chamber and the receiving chamber. In each case,the mechanical grasping member may be replaced with a suction head. Inaddition to or in place of the arm, retractable conveyor belts may bepositioned to carry the substrate away from the holder. A turntable orother rack system carrying different substrates could be employed.Numerous additional variations or alternatives to the foregoing meanswill be readily apparent to those skilled in the art. Accordingly, theinvention is defined by the following claims, with equivalents of theclaims to be included therein.

That which is claimed is:
 1. A spin coating method, comprising:applying a carbon dioxide liquid containing a coating component to a surface portion of a substrate; and then rotating said substrate about an axis to distribute said carbon dioxide liquid on said substrate.
 2. A method according to claim 1, wherein said rotating step is carried in an atmosphere comprising carbon dioxide at a pressure greater than atmospheric pressure.
 3. A method according to claim 1, wherein said rotating step is carried out in an atmosphere comprising carbon dioxide at a pressure of 10 to 10,000 psi.
 4. A method according to claim 1, wherein said rotating step is carried out in an atmosphere comprising carbon dioxide, said method further comprising the step of:maintaining a differential partial pressure of carbon dioxide between said carbon dioxide liquid and said atmosphere of between about 10 and 400 mm Hg.
 5. A method according to claim 4, wherein said applying and rotating steps are carried out in an enclosed chamber, and said maintaining step is carried out by:providing a reservoir of carbon dioxide liquid within said enclosed chamber; and controlling the temperature difference between the carbon dioxide liquid in said reservoir and the carbon dioxide liquid applied to said substrate surface portion.
 6. A method according to claim 4, wherein said applying and rotating steps are carried out in an enclosed chamber, and said maintaining step is carried out by:supplying an inert gas to said enclosed chamber.
 7. A method according to claim 4, wherein said applying and rotating steps are carried out in an enclosed chamber, said enclosed chamber formed by a pressure vessel, and said maintaining step is carried out by:cooling a wall portion of said pressure vessel.
 8. A method according to claim 1, wherein said liquid comprises a mixture of carbon dioxide and a compound selected from the group consisting of polymers, adhesion promoters, and antireflective coatings.
 9. A method according to claim 8, wherein said mixture is selected from the group consisting of solutions, dispersions, and emulsions.
 10. A method according to claim 8, wherein said compound is a fluoropolymer.
 11. A method according to claim 8, wherein said compound is a fluoroacrylate polymer.
 12. A method according to claim 11, wherein said mixture is a solution.
 13. A method according to claim 8, wherein said mixture comprises a viscosity modifier.
 14. A method according to claim 8, wherein said mixture comprises a surface-tension modifier.
 15. A method according to claim 1, wherein said substrate surface portion is planar.
 16. A method according to claim 1, wherein said liquid comprises a mixture of carbon dioxide and a sol-gel precursor. 